Abstract
Large amounts of the carbon-isotope 14C, entering
Earth’s carbon cycle, were produced in the atmosphere by atomic bomb
tests in the 1950s and 1960s. Here, we forced the ocean and land
components of the Community Earth System Model with atmospheric
14CO2 over the historical period to
constrain overturning time scales and fluxes. The uptake of bomb
14C by the land model is lower than observation-based
estimates. This mismatch is likely linked to too low
14C uptake by vegetation as the model overestimates
14C/C ratios of modern soils indicating model biases
in forest productivity or wood carbon allocation and turnover. The ocean
model matches the observation-based global bomb 14C
inventories when applying the Large and Yeager wind data and the
quadratic relationship between gas transfer piston velocity and wind
speed of Wanninkhof, 2014. However, ocean bomb 14C
inventories are underestimated in simulations with winds from the
Japanese Reanalysis Project, calling for an upward revision of the
piston velocity by 15% for this wind product. The sum of ocean, land,
and atmospheric bomb 14C inventory changes is lower in
the 1960s than reconstructed bomb 14C production,
likely due to uncertainties in the observational production and
atmospheric records and too low land model 14C uptake.
Simulated natural radiocarbon ages in the deep ocean are many centuries
older than data-based estimates, indicating too slow deep ocean
ventilation. Our study suggests that 14C observations
are key to constrain carbon fluxes and transport timescales within Earth
system models.